Replace deleted certificate from mail server
I originally had a self signed certificate for my postfix server. I purchased one and Thunderbird accepted it with error as it was for the domain (cert was for "kites.org"), not the host that was the mail server (kumo.kites.org). I reconfigured the certificate and re-installed it in postfix and dovecot, then deleted the self signed original and the second domain based certificate. However, the new certificate doesn't show up. It shows up when testing the server and Thunderbird connects with ssl just fine.
Opaite Mbohovái (1)
You can try going to https://www.ssllabs.com/ to check SSL certificate installation issues and fix them. They'll give your configuration a quick look and see if they can spot your issue.
Or, you could try generating a new SSL for your domain: from SSL For Free ... I use the "Let's Encrypt" authority for serving my web pages as HTTPS ... but I do not really know If a MX record can be set up after you install one of these. I'll bet it CAN.
These genuine SSL Certs are indeed FREE: https://www.sslforfree.com/
Many Hosting Plans that have a cPanel DO Have Support for Free "Let's Encrypt" Certificates.
Look in your cPanel for the "Security" section. (See screenshots below) Try "SSL/TSL Status" and "SSL/TSL" sub panels. You can paste in a certificate in the Certificates (CRT) Area ... Generate, view, upload, or delete SSL certificates.
I think it was when I looked in the "SSL/TSL Status" panel for the first time that I found shiny new SSL Certificates waiting for me to use them. I did "HSTS" to force HTTPS on all pages ... after I merely flipped the switch, to activate them ... and I was HTTPS from then on.
Moambuepyre